A typical disc drive includes one or more discs mounted for rotation on a hub or spindle. A typical disc drive also includes a transducer supported by a hydrodynamic air bearing which flies above each disc. The transducer and the hydrodynamic air bearing are collectively referred to as a data head. A drive controller is conventionally used for controlling the disc drive based on commands received from a host system. The drive controller controls the disc drive to retrieve information from the discs and to store information on the discs.
In one conventional disc drive, an electromechanical actuator operates within a negative feedback, closed-loop servo system. The actuator moves the data head radially over the disc surface for track seek operations and holds the transducer directly over a track on the disc surface for track following operations.
Information is typically stored in concentric tracks on the surface of the discs by providing a write signal to the data head to write information on the surface of the disc representing the data to be stored. In retrieving data from the disc, the drive controller controls the electromechanical actuator so that the data head flies above the disc and generates a read signal based on information stored on the disc. The read signal is typically conditioned and then decoded by the drive controller to recover the data.
A typical read channel includes the data head, preconditioning logic (such as preamplification circuitry and filtering circuitry), a data detector and recovery circuit, and error detection and correction circuitry. The read channel is typically implemented in a drive controller associated with the disc drive.
In disc drives, it is important that the error rate per number of bits recorded (the bit error rate) be maintained at a relatively low level. In order to improve bit error rate performance in disc drives, or in order to increase the linear recording density in disc drives, maximum likelihood sequence detection (MLSD) methods are desired. Such methods can be implemented using the well known Viterbi algorithm. However, a direct implementation of an MLSD method is very costly. For example, the channel response after forward filtering is typically quite long, and may contain ten or more terms. Thus, a Viterbi detector would require 2.sup.10-1 sates, which is impractically complex. Therefore, other techniques have been investigated which tend to reduce complexity yet still provide results which approach those of direct MLSD methods.
One such technique is to apply the Viterbi algorithm to a reduced number of terms by cancelling some of the terms with feedback. For example, by cancelling all but two terms (and including the main cursor) allows the Viterbi detector to have only four states. Such detectors are referred to as reduced state sequence estimators (RSSE).
Another technique is to choose a channel response target which is not a perfectly whitened target, but which has a fewer number of terms. In such systems, partial response (PR) targets have been developed. Among those targets is one referred to as enhanced extended partial response maximum likelihood (E.sup.2 PRML) target. At high recording densities, it has been observed that for certain high order partial response channels (such as the E.sup.2 PRML) channel, the dominant error events (the difference between two input sequences) encountered with detectors used with such partial response targets are generally of the form .+-.(2,-2,2). Such errors are typically caused when a tribit is shifted by one sample time, or when a quadbit is mistaken as a dibit or vise versa.